Protection of magnesium and magnesium alloys



Dec, w, 1940. H. SUTTON ET AL PROTECTION OF MAGNESIUM AND MAGNESIUM ALLOYS Filed Dec. 17, 1938 INVENTORS HSuTToN m L.F. LE BRocQ WIW 7 M) Arromvevs Patented Dec. 10, 1940 UNITED STATES PROTECTION OF MAGNESIUM AND MAGNESIUM ALLOYS Hubert Sutton and Laurence Francis Le Brocq, South Farnborough, England Application December 17, 1938, Serial No. 246,476 In Great Britain December 23, 1937 8 Claims.

This invention relates to the protection of magnesium and magnesium alloys against corrosion, and particularly to the production of a suitable coating on the surface of the metal by immersion in a solution which can be used cold, 1. e. it is not necessary to supply heat to the solution from an external source such as by immersion electric resistance heaters or by gas burners. The invention relates to solutions which contain chromic acid ions and also one or more salts which may be called activator salts by reason of the more rapid action on the metal which occurs when they are introduced.

The solution, to be really satisfactory, should have at least' the following characteristics:

(1) It should be capable of producing an even coating providing good protection against cor rosion and which is firmly adherent to the metal with consistent results.

(2) The coating should be produced within a reasonably short period of time for obvious commercial reasons. A bath that requires an immersion period of several hours is too slow for commercial work and such a bath would really require heating or acidifying to shorten the immersion period.

(3) The coating should not be produced too quickly since this militates against firm adherence of the coating. Usually a coating produced in a period of ten minutes or less can be rubbed oif by the fingers. An immersion period of the order of half-an-hour is usually suitable.

(4) The solution should be capable of retaining its ability to produce good coatings at the required speed for fairly considerable periods, so that reasonable quantities of metal can be treated before the bath loses its effectiveness.

The time required for the formation of a protective coating of given thickness and density depends, with a bath of otherwise identical composition, on the pH value and the temperature of the bath, in the sense that with lower temperatures, e. g. in the cold, a lower pH value is required than with elevated temperatures; the speed of the bath is increased by lowering its pH value as well as by raising its temperature. On the other hand, the period over which a bath retains its desirable characteristics as aboveset forth, depends largely on the variation of the hydrogen ion concentration therein during use of the bath, which variation is due to progressive reaction of the constituents of the bath, and particularly its CIO3 content, with the metal to be treated. For example, in order to obtain desirable protective coatings or films within a suitable period of immersion, it is desirable with most cold solutions to maintain the hydrogen ion concentration such that the pH value is between 2 and 5, but if the solution, because of its particular composition, e. g. the activator salt used, or because of the application of electric current to the metal being treated. has an abnormally fast attack on the metal a higher pH value may be necessary. If hydrogen ion concentration curves are prepared in which pH valuesare plotted against weight of alkali added to the bath, it can be seen that solutions containing also certain activator salts produce curves which remain in the required pH range over a considerably greater variation in the weight of added alkali than is possible with other activator salts. Thus it will be seen that as metal is treated and as a consequence the bath becomes more alkaline, the resulting change in acidity will not be sufiicient to prevent the bath from acting efficiently for considerable periods. Curves for potassium, magnesium and nickel salts for instance, vary rapidly in the pH range 2 to 5, but have a considerably smaller inclination in the pH range 5 to 7, which however is a range which will produce coatings in the cold only after extended periods of immersion, and thus are more suitable for use as hot baths.

In the accompanying drawing the abscissae represent the number of gramme equivalents of caustic potash added to gramme equivalent of chromic anhydride, gramme equivalent of potassium dichromate, and one gramme equivalent of a salt other than a chromate or dichromate, all dissolved in two liters of water. The ordinates represent the corresponding pH values of the solution as determined with a glass electrode. The chromic acid was only used for obtaining the very acid parts of the curves and for the remainder of the curves the solutions can be regarded as containing no chromic acid but 1 gramme equivalent of potassium dichromate.

The curves are marked 1 to 6 respectively and represent solutions composed as follows:

Salt added to mixture of chromic anhydride and potassium dicbromate Curve No.

None.

Magnesiumsulphate.

Nickel sulphate. Potash alum.

. Chrome alum.

Iron alum.

means that the bath would remain effective for the treatment of a correspondingly large quantity of metal while still producing the required coating thereon fast enough. It can be seen from curves 5 2 and 3, however, that if the initial composition of the corresponding baths corresponds to a pH value of 3, then such baths will rapidly acquire a pH value of over 4 so that after the treatment of only a comparatively small quantity of metal the required speed of the bath is lost.

The curves 4, 5, and 6, all have parts in the effective pH range which do not undergo a pH variation of more than 2 for an addition of 0.4 gramme equivalent of potassium hydroxide: for

the purpose of the present invention the solution must have a curve having a part (in the pH range which can produce a satisfactory coating) which has a gradient at least as low as indicated by the foregoing statement.

We have previously proposed in our British Patent No. 331,853 to produce coatings on magnesium and magnesium alloys with the aid of a heated neutral or acid aqueous solution containing chromates and dichromates of an alkali metal together with a compound or salt of aluminium. However, the acidity of the bath with which we were then concerned was close to the neutral range, as can be seen from the suggested composition of the bath and from the fact that it was proposed to use this bath at a temperature of 95-100 0., and even then required a period of several hours of immersion to produce the necessary coating.

According to the present invention the constituents of the bath and their relative proportions will be selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acid content in the pH range which is required for producing a protective adherent film in a period not exceeding two hours (or preferably one hour). Heating of the bath according to the present invention must be avoided since the speed of action on the metal Would be too rapid and to avoid this the bath would have to be made more alkaline which may precipitate the constituents of the bath. For commencement of use of the bath a pH value should be chosen which is on the lower side of the flat part of the relative hydrogen ion concentration curve so that the bath can continue to operate over the whole of the part of slight inclination (i. e. the part of gradual pH change) of the curve until the bath becomes too near the neutral range. For the purpose of the present invention it is preferable to employ aluminium alums or other aluminium salts such as aluminium sulphate or nitrate (the chloride not being very satisfactory) as the said part of the hydrogen ion concentration curve for aluminium salts remains longer in the required pH range than the curves produced when using chromium or iron alums. Moreover, when using heavy metal salts there is always a tendency to produce heavy metal deposits in the protective coating which give rise to corrosion. Chromium and iron alums have, however, hydrogen ion concentration curves with parts of slight inclination in the required pH range. The cold bath according to the present invention should be used at the pH range at which the aluminium salts are actually in solution so that no bloom will be produced on the surface of the metal.

If desired chlorides may be added to maintain the alumina in colloidal solution or suspension when the bath has been'used for some time.

In the drawing, the compositions at which deposition of metallic compounds occurs are indicated by the parts of the curves on the alkaline side of the positions marked by arrows. When using aluminium alums or other aluminium salts 5 the alumina will precipitate or form a bloom" on the surface of the metal after use of the bath has caused its pH value to exceed about 4, and if this occurs the addition of an acid will be sufiicient to make the bath usable for a further 10 period. The acid may be chromic or sulphuric and the amount is preferably sufiicient to cause the pH value to return to about 3 or to the value where the pH curve assumes a small inclination. When making up the bath the relative proporl5 tions of the constituents will be selected to produce the required hydrogen ion concentration. The bath can be otherwise controlled, for example by periodically ascertaining its acidity by titration or otherwise. 20

The chromic acid ions may be introduced by adding potassium dichromate, but other chromates or dichromates of the alkali metals or ammonia, magnesium, aluminium and zinc may be used. 25

Certain salts can be selected with appropriate degrees of solubility so that a concentrated and sometimes even a saturated solution thereof will have the required hydrogen ion concentration, and if some solid salts are always present, the 30 bath will tend to remain longer in the required pH range. Thus potassium dichromate and potassium or ammonium alum will produce such a solution. These salts can be kept in perforated metal containers in the bath so that it can be 35 readily seen whether any solid salts remain. The necessary period of immersion can be varied not only by changing the hydrogen ion concentration but also by varying the relation between the amounts of dichromate to activator salts 40 present.

It appears that the addition of a permanganate,

e. g. potassium permanganate, to the bath, has a beneficial effect on the protective coatings which then appear to be more resistant to corrosion. 45

Agitation of the bath by a mechanical stirrer or otherwise is also of advantage.

If desired the bath may be used as an electrolytic bath by suspending the metal in the bath and connecting it to the metal of the bath or 50 other electrode or by otherwise supplying electric current thereto.

If, after a lengthy period of working, the bath eventually becomes sluggish in action, its activity may conveniently be restored by the addition 55 of chromic acid or the acids corresponding to the salts originally added to the bath. A convenient arrangement is to add a mixture of chromic acid and another acid in such relative proportions that the proportion of acid radicles originally 60 present in the bath is not disturbed.

As an example of the process, magnesium alloy parts containing 57% of aluminium, about 2% of zinc, and about 0.5% of manganese, may be cleaned in an acid or an alkaline cleaner, washed in water, and then transferred to an aqueous solution saturated with potassium dichromate and with potash alum or ammonia alum at a temperature of 20 C. The parts may be left in the solution for periods from thirty minutes up 70 to several hours without serious loss of metal, under which conditions they become coated with a brown adherent protective film. In this example both the dichromate and the salts other than dichromates are present as saturated solutions, 76

and hence the concentration of all the constituents of the bath may be maintained by the simple addition of further amounts of the various salts or of the above cited substances without affecting the working properties of the bath.

A solution which has given good results has the following composition:

Grammes Potassium dichromate 10 Potassium alum 10 Potassium permanganate 5 Water The most desirable pH range for this solution appears to be 3-3.9.

We claim:

1. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solution containing chromic acid ions and ions of an activator salt selected from the group consisting of aluminium salts and alums and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acidity in the pH range which is required for producing a. protective adherent film in the cold in a period not exceeding two hours, the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12 grammes of potassium hydroxide to 1 litre of the solution.

2. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solution containing chromic acid ions and ions of an activator salt selected from the group consisting of aluminium salts and alums and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acidity in the pH range 2-5 in which range it is capable of producing a protective adherent film in the cold in a period not exceeding two hours without the use of electric current, the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12 grammes of potassium hydroxide to 1 litre of the solution.

3. A process for the protection of magnesium and magnesium alloys wherein the metal is im mersed in a cold aqueous solution containing .chromic acid ions and aluminium alum and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acidity in the pH range which is required for producing a protective adherent film in the cold in a period not exceeding two hours, the solution being such that its pH value does not change more than '2 over part of said range for the addition of 12 grammes of potassium. hydroxide to 1 litre of the solution.

4. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solution containing chromic acid ions and aluminium alum and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acidity in the pH range 2-5 in which range it is capable of producing a protective adherent film in the cold in a period not exceeding two hours without the application of electric current, the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12' grammes of potassium hydroxide to 1 litre of the solution.

5. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solution containing chromic acid ions and ions of an activator salt from the group consisting of chromium and iron alum and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concen tration curve which remains for a considerable variation of added alkali or acid content in the pH range which is required for producing a protective adherent film in the cold in a period not exceeding two hours, the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12 grammes of potassium hydroxide to 1 litre of the solution.

6. A process for the protection of magnesium and magnesium alloys wherein the metal is im-' mersed in a cold aqueous solution containing a chromium salt from the group consisting of alkali metal and ammonium salts and containing ions of one or more activator salts from the group consisting ofalums and aluminium salts grammes of potassium hydroxide to 1 litre of A the solution.

7. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solutioncontaining a chromium salt from the group consisting of alkali metal and ammonium salts, and aluminium alum, and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of, acidity in the pH range which is required for producing a protective adherent film in the cold in a period not exceeding two hours,

the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12 grammes of potassium hydroxide to 1 litre of the solution.

8. A process for the protection of magnesium and magnesium alloys wherein the metal is immersed in a cold aqueous solution containing a chromium salt from the group consisting of alkali metal and ammonium salts, and aluminium.

alum, and in which the constituents of the solution and their relative proportions are selected so that the solution has a hydrogen ion concentration curve which remains for a considerable variation of acidity in the pH range 2-5 in which it is capable of producing a protective adherent film in the cold in a period not exceeding two hours, the solution being such that its pH value does not change more than 2 over part of said range for the addition of 12 grammes of potassium hydroxide to 1 litre of the solution. I-IUBERT SUTTON. LAURENCE FRANCIS LE BROCQ. 

